RFC 3034
Use of Label Switching on Frame Relay Networks Specification
RFC 3034 🎀 Use of Label Switching on Frame Relay Networks
okay bestie, here's the header tea ☕💅
- RFC number: 3034
- Title: Use of Label Switching on Frame Relay Networks, Specification
- Authors: A. Conta (Transwitch Corporation), P. Doolan (Ennovate), A. Malis (Vivace Networks, Inc.)
- Category: Standards Track
- Date: January 2001
- Obsoletes / Updates: the title block doesn't list anything this RFC obsoletes or updates 🤷♀️
Status of This Memo ✨
bestie, this doc is an Internet standards track protocol for the Internet community, and it's literally asking for discussion and suggestions 🗣️ Check the current edition of "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited 💌
Copyright Notice 🌈
Copyright (C) The Internet Society (2001). All Rights Reserved. The Internet Society holds the copyright, and all rights are reserved 🔒
Abstract 🦋
this doc defines the model and generic mechanisms for Multiprotocol Label Switching on Frame Relay networks 🌸 It also extends and clarifies parts of the Multiprotocol Label Switching Architecture described in [ARCH] and the Label Distribution Protocol (LDP) described in [LDP], specifically for Frame Relay Networks. MPLS lets Frame Relay Switches act as Label Switching Routers (LSRs) 🧚
Table of Contents 📑
1. Introduction................................................2
2. Terminology.................................................3
3. Special Characteristics of Frame Relay Switches.............4
4. Label Encapsulation.........................................5
5. Frame Relay Label Switching Processing......................6
5.1 Use of DLCIs..............................................6
5.2 Homogeneous LSPs..........................................7
5.3 Heterogeneous LSPs........................................7
5.4 Frame Relay Label Switching Loop Prevention and Control...7
5.4.1 FR-LSRs Loop Control - MPLS TTL Processing.............7
5.4.2 Performing MPLS TTL calculations.......................8
5.5 Label Processing by Ingress FR-LSRs......................12
5.6 Label Processing by Core FR-LSRs.........................12
5.7 Label Processing by Egress FR-LSRs.......................13
6. Label Switching Control Component for Frame Relay.........13
6.1 Hybrid Switches (Ships in the Night) ...................14
7. Label Allocation and Maintenance Procedures ..............15
7.1 Edge LSR Behavior........................................15
7.2 Efficient use of label space-Merging FR-LSRs.............18
7.3 LDP message fields specific to Frame Relay...............19
8. Security Considerations .................................21
9. Acknowledgments .........................................21
10. References ..............................................22
11. Authors' Addresses ......................................23
12. Full Copyright Statement ................................241. Introduction 🦋
the Multiprotocol Label Switching Architecture is written up in [ARCH] 📖 It's possible to use Frame Relay switches as Label Switching Routers. These Frame Relay switches run network layer routing algorithms (like OSPF, IS-IS, etc.), and their forwarding is based on what those routing algorithms come up with. No specific Frame Relay routing is needed 🙌
when a Frame Relay switch does label switching, the top (current) label, which is what forwarding decisions are based on, rides in the DLCI field of the Frame Relay data link layer header of a frame 💌 Any extra info that goes along with the top (current) label, but isn't processed by Frame Relay switching, plus the other labels if the packet has more than one, gets carried in the generic MPLS encapsulation defined in [STACK] 🫶
Frame Relay permanent virtual circuits (PVCs) could be set up to carry label switching based traffic ✨ The DLCIs would be used as MPLS Labels, the Frame Relay switches would become Frame Relay Label Switching Routers, and the MPLS traffic would be encapsulated according to this specification and forwarded based on network layer routing information 🧚
The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED, SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as defined in RFC 2119 🎀
these capital-letter keywords are the important ones, so read them with care 👀
This document is a companion document to [STACK] and [ATM] 💜
2. Terminology 📚
bestie, these are the building blocks you'll see everywhere, so let's get them locked in 🔒
LSR
A Label Switching Router (LSR) is a device that implements the label switching control and forwarding components described in [ARCH] 🖥️
LC-FR
A label switching controlled Frame Relay (LC-FR) interface is a Frame Relay interface controlled by the label switching control component. Packets going over one of these carry labels in the DLCI field 🏷️
FR-LSR
A FR-LSR is an LSR with one or more LC-FR interfaces, and it forwards frames between two of those interfaces using labels carried in the DLCI field 🔁
FR-LSR domain
A FR-LSR domain is a set of FR-LSRs that are mutually interconnected by LC-FR interfaces 🌐
Edge Set
The Edge Set of an FR-LSR domain is the set of LSRs that are connected to the domain by LC-FR interfaces 🚪
Forwarding Encapsulation
The Forwarding Encapsulation is the type of MPLS encapsulation (Frame Relay, ATM, Generic) of a packet that decides how that packet gets forwarded over MPLS. If the packet is forwarded based on the network layer (IP, etc...)header instead, it's the network layer encapsulation 🎯
Input Encapsulation
The Input Encapsulation is the type of MPLS encapsulation (Frame Relay, ATM, Generic) of a packet when it arrives on an LSR's interface. If the packet has no MPLS encapsulation, it's the network layer (IP, etc...)encapsulation instead 📥
Output Encapsulation
The Output Encapsulation is the type of MPLS encapsulation (Frame Relay, ATM, Generic) of a packet when it gets sent out an LSR's interface. If the packet has no MPLS encapsulation, it's the network layer (IP, etc...)encapsulation instead 📤
Input TTL
The Input TTL is the MPLS TTL of the top of the stack when a labeled packet is received on an LSR interface. If the packet isn't labeled, it's the network layer (IP) TTL instead ⏳
Output TTL
The Output TTL is the MPLS TTL of the top of the stack when a labeled packet is transmitted on an LSR interface. If the packet isn't labeled, it's the network layer (IP) TTL instead ⌛
Additionally, this document uses terminology from [ARCH] 🫶
3. Special characteristics of Frame Relay Switches 🍓
the label switching architecture is pretty flexible when it comes to LSR implementation, but a FR-LSR is limited by the capabilities of its (possibly already existing) hardware 🛠️ It's also limited by restrictions on things like frame format, imposed by the Multiprotocol Interconnect over Frame Relay [MIFR] or Frame Relay standards [FRF], etc.... Because of all these constraints, some special procedures are required for FR-LSRs 😤
some of the key features of Frame Relay switches that affect how they act as LSRs are:
- the label swapping function is performed on fields (DLCI) in the frame's Frame Relay data link header. This dictates the size and placement of the label(s) in a packet 🏷️ The size of the DLCI field can be 10 (default) or 23 bits, and it can span two or four bytes in the header.
- there is generally no capability to perform a 'TTL-decrement' function like the one performed on IP headers in routers 🙈
- congestion control is performed by each node based on parameters that are passed at circuit creation. Flags in the frame headers may be set as a consequence of congestion, or of exceeding the contractual parameters of the circuit 🚦
- although in a standard switch it may be possible to configure multiple input DLCIs to one output DLCI, resulting in a multipoint-to-point circuit, multipoint-to-multipoint VCs are generally not fully supported 💔
this document describes ways of applying label switching to Frame Relay switches that work within these constraints 💖
4. Label Encapsulation 🎀
by default, all labeled packets should be transmitted with the generic label encapsulation defined in [STACK], using the frame relay null encapsulation mechanism 🎁
0 1 (Octets)
+-----------------------+-----------------------+
(Octets)0 | |
/ Q.922 Address /
/ (length 'n' equals 2 or 4) /
| |
+-----------------------+-----------------------+
n | . |
/ . /
/ MPLS packet /
| . |
+-----------------------+-----------------------+"n" is the length of the Q.922 Address, which can be 2 or 4 octets 📏
the Q.922 [ITU] representation of a DLCI (in canonical order, where the first bit is stored in the least significant, i.e., the right-most bit of a byte in memory) [CANON] looks like this 🧠
7 6 5 4 3 2 1 0 (bit order)
+-----+-----+-----+-----+-----+-----+-----+-----+
(octet) 0 | DLCI(high order) | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+
1 | DLCI(low order) | 0 | 0 | 0 | 1 |
+-----+-----+-----+-----+-----+-----+-----+-----+
10 bits DLCI
7 6 5 4 3 2 1 0 (bit order)
+-----+-----+-----+-----+-----+-----+-----+-----00
(octet) 0 | DLCI(high order) | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----
1 | DLCI | 0 | 0 | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+
2 | DLCI | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+
3 | DLCI (low order) | 0 | 1 |
+-----+-----+-----+-----+-----+-----+-----+-----+
23 bits DLCIthe frame relay null encapsulation implies that labels implicitly encode the network protocol type 🕵️♀️
rules about building the label stack, and the error messages returned to the frame source, are also described in [STACK] 📜
the generic encapsulation contains "n" labels for a label stack of depth "n" [STACK]. The top stack entry carries meaningful values for the EXP, S, and TTL fields [STACK], but not for the label. The label is instead carried in the DLCI field of the Frame Relay data link header, encoded in Q.922 [ITU] address format 🌸
5.1. Use of DLCIs 🎀
Okay so label switching works by attaching labels to routes and then using that label value to forward packets, including working out what the replacement label should be. 🦋 See [ARCH] for the full scoop. In a FR-LSR, the top (current) MPLS label rides in the DLCI field of the Frame Relay data link layer header of the frame. The top label also implicitly tells you the network protocol type. 💅
For two connected FR-LSRs, a full-duplex connection MUST be available for LDP. Not me being dramatic, but this is literally a must-have. 😭 The DLCI for the LDP VC gets its value through configuration, just like how you'd configure the DLCI used to run IP routing protocols between the switches. ✨
Apart from that one configured value, the DLCI values used for MPLS in the two directions of the link may be treated as belonging to two independent spaces. Basically, VCs may be half-duplex, with each direction getting its own DLCI. 🌈
The allowable ranges of DLCIs, the size of DLCIs, and the support for VC merging MUST be communicated through LDP messages. 🫶 Also, the range of DLCIs used for labels depends on the size of the DLCI field, so keep that in mind, bestie. 🧚
5.2. Homogeneous LSPs 🍓
If <LSR1, LSR2, LSR3> is an LSP, it's possible that LSR1, LSR2, and LSR3 all use the same encoding of the label stack when transmitting packet P from LSR1, to LSR2, and then to LSR3. That's giving consistency. ✨ An LSP like this is called homogeneous. 👑
5.3. Heterogeneous LSPs 🌸
If <LSR1, LSR2, LSR3> is an LSP, it's possible that LSR1 uses one encoding of the label stack when transmitting packet P to LSR2, but LSR2 uses a different encoding when transmitting a packet P to LSR3. The MPLS architecture generally supports LSPs with different label stack encodings on different hops. 🦋
When a labeled packet is received, the LSR must decode it to find the current value of the label stack. Then it must operate on the label stack to determine the new label value of the stack, and then encode the new value appropriately before transmitting the labeled packet to its next hop. 💖
Naturally, there will be MPLS networks that mix Frame Relay switches operating as LSRs with other LSRs that use other MPLS encapsulations, like the Generic (MPLS shim header) or ATM encapsulation. 🌈 In these networks, some LSRs have Frame Relay interfaces as well as MPLS Generic ("MPLS Shim") interfaces. This is one example of an LSR with different label stack encodings on different hops of the same LSP. 🔥 Such an LSR may swap off a Frame Relay encoded label on an incoming interface and replace it with a label encoded into a Generic MPLS (MPLS shim) header on the outgoing interface. 🧚
5.4. Frame Relay Label Switching Loop Prevention and Control 🔥
FR-LSRs SHOULD operate on loop free FR-LSPs or LSP Frame Relay segments. So, FR-LSRs SHOULD use loop detection, and they MAY use loop prevention mechanisms as described in [ARCH], and [LDP]. 🙈
5.4.1. FR-LSRs Loop Control - MPLS TTL processing 💅
The MPLS TTL encoded in the MPLS label stack is a mechanism used to:
- (a) suppress loops;
- (b) limit the scope of a packet.
When a packet travels along an LSP, it should emerge with the same TTL value it would have had if it had traversed the same sequence of routers without being label switched. ✨ If the packet travels along a hierarchy of LSPs, the total number of LSR-hops traversed should be reflected in its TTL value when it emerges from the hierarchy of LSPs [ARCH]. 👑
The initial value of the MPLS TTL is loaded into a newly pushed label stack entry from the previous TTL value. That previous value comes either from the network layer header when no previous label stack existed, or from a pre-existent lower level label stack entry. 🦋
A FR-LSR switching same level labeled packets does not decrement the MPLS TTL. A sequence of such FR-LSRs is called a "non-TTL segment". 🌸
When a packet emerges from a "non-TTL LSP segment", it should however reflect in the TTL the number of LSR-hops it traversed. In the unicast case, this can be achieved by propagating a meaningful LSP length or LSP Frame Relay segment length to the FR-LSR ingress nodes. That lets the ingress decrement the TTL value before forwarding packets into a non-TTL LSP segment [ARCH]. 🍓
⚠️ Heads up, this one's serious: when an ingress FR-LSR decrements the MPLS TTL and sees that a particular packet's TTL will expire before the packet reaches the egress of the "non-TTL LSP segment", the FR-LSR MUST not label switch the packet. Instead, it follows the specifications in [STACK] to try to return an error message to the packet's source:
- it treats the packet as an expired packet and returns an ICMP message to its source.
- it forwards the packet, as an unlabeled packet, with a TTL that reflects the IP (network layer) forwarding.
If the incoming TTL is 1, only the first option applies. 🙈
In the multicast case, a meaningful LSP length or LSP segment length is propagated to the FR-LSR egress node. That lets the egress decrement the TTL value before forwarding packets out of the non-TTL LSP segment. 🫶
5.4.2. Performing MPLS TTL calculations 🧮
The calculation applied to the "input TTL" that yields the "output TTL" depends on (i) the "input encapsulation", (ii) the "forwarding encapsulation", and (iii) the "output encapsulation". 💖 The relationship among (i), (ii), and (iii) can be defined as a function "D" of "input encapsulation" (ie), "forwarding encapsulation" (fe), and "output encapsulation" (oe). So the calculation applied to the "input TTL" to yield the "output TTL" can be described as:
output TTL = input TTL - D(ie, fe, oe)or, in a brief notation:
output TTL = input TTL - dwhere "d" has three possible values: "0", "1", or "the number of hops of the LSP segment". 🌈
For unicast transmission:
+================+=================+=================+=================+
| | Type of | Type of | Type of |
| d | Input | Forwarding | Output |
| | Encapsulation | Encapsulation | Encapsulation |
+================+=================+=================+=================+
| 0 | Frame Relay | Frame Relay | Frame Relay |
+----------------+-----------------+-----------------+-----------------+
| 1 | any | Generic MPLS | Generic MPLS |
+----------------+-----------------+-----------------+-----------------+
| number of hops | | Generic MPLS | |
| of | any | or | Frame Relay |
| LSP segment | |IP(network layer)| |
+================+=================+=================+=================+The "number of hops of the LSP segment" is the value of the "hop count" attached to the label used when the packet is forwarded. This applies if LDP [LDP] provided such a "hop count" value when it distributed the label for the LSP, meaning the LDP message had a "hop count object". If LDP didn't provide a "hop count", or it provided an "unknown" value, the default value of the "number of hops of the segment" is 1. ✨
When sending a label binding upstream, the "hop count" associated with the corresponding binding from downstream, if it's different from the "unknown" value, MUST be incremented by 1. The result is then transmitted upstream as the hop count associated with the new binding (the "unknown" value is transmitted unchanged). 👑 If the new "hop count" value exceeds the "maximum" value, the FR-LSR MUST NOT pass the binding upstream. Instead, it MUST send an error upstream [LDP][ARCH]. 😭
For multicast transmission:
+================+=================+=================+=================+
| | Type of | Type of | Type of |
| d | Input | Forwarding | Output |
| | Encapsulation | Encapsulation | Encapsulation |
+================+=================+=================+=================+
| 0 | Frame Relay | Frame Relay | Frame Relay |
+----------------+-----------------+-----------------+-----------------+
| | | Generic MPLS | |
| 1 | any | or | Frame Relay |
| | |IP(network layer)| |
+----------------+-----------------+-----------------+-----------------+
| number of hops | | Generic MPLS | |
| of | Frame Relay | or | any |
| LSP segment | |IP(network layer)| |
+================+=================+=================+=================+Referring to the "forwarding encapsulation" with the abbreviation "I" for IP (network layer), "G" for Generic MPLS, and "F" for Frame Relay MPLS. An LSR interface gets the abbreviation "i" if the input or output encapsulation is IP and no MPLS encapsulation, "g" when the input or output MPLS encapsulation is Generic MPLS, "f" when it's Frame Relay, and "a" when it's ATM. Then, if we consider the symbols "iIf", "gGf", "fFf", etc. as LSRs with input, forwarding and output encapsulations as referred above, the following describes examples of TTL calculations for the Homogeneous and Heterogeneous LSPs discussed in previous sections. 🧚
Homogeneous LSP
---------------
IP_ttl = n IP_ttl=mpls_ttl-1 = n-6
--------->iIf fIi--------->
| mpls_ttl = n-5 ^
| |
number of hops 1| Frame Relay |5
| |
V 2 3 4 |
fFf--->fFf--->fFf--->fFf
"iIf" is "ingress LSR" in Frame Relay LSP and
calculates: mpls_ttl = IP_TTL - number of hops = n-5
"fIi" is "egress LSR" from Frame Relay LSP, and
calculates: IP_ttl = mpls_ttl-1 = n-6 Heterogeneous LSP
-----------------
ingress LSR egress LSR
IP_ttl = n IP_ttl = n - 15
links LAN PPP FR ATM PPP FR LAN
--->iIg-->gGg-->gGf fGa aGg-->gGf fGg-->gIi--->
hops 1 2 | 6 | | 9 | 10 | 13 ^ 14 15
|1 4| |1 3| |1 3|
V 2 3 | V 2 | V 2 |
fFf-->fFf-->fFf aAa-->aAa fFf-->fFf
mpls_ttl
n-1 n-2 (n-2)-4=n-6 (n-6)-3=n-9 n-10 n-13 n-14
"iIg" is "ingress LSR" in LSP; it calculates: mpls_ttl=n-1
"gGf" is "egress LSR" from Generic MPLS segment, and
"ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-6
"fGa" "egress LSR" from Frame Relay segment, and
"ingress LSR" in ATM segment and calculates: mpls_ttl=n-9
"gGf" is "egress LSR" from Generic MPLS segment, and
"ingress LSR" in Frame Relay segment and calculates: mpls_ttl=n-13
"fGg" is "egress LSR" from Frame Relay segment, and
ingress LSR" in Generic MPLS segment and calculates: mpls_ttl=n-14"gIi" is "egress LSR" from LSP and calculates: IP_ttl=n-15 💅
And further examples, bestie 🦋
Frame Relay Unicast -- TTL calculated at ingress
(ingress LSR) 1 2 3 4
x--->---+--->---+--->>--+-->>---x (egress LSR)
o.ttl=i.ttl-4 | 2 3
^
hops 1|
|
x (ingress LSR)
o.ttl=i.ttl-3 Frame Relay Multicast -- TTL calculated at egress
(egress LSR)x o.ttl=i.ttl-3
hops |
^3
(ingress LSR) | o.ttl=i.ttl-4
x--->---+--->---+--->---+--->---x (egress LSR)
1 2 3 45.5 Label Processing by Ingress FR-LSRs
okay so when a packet first walks into an MPLS domain 🌸 it gets forwarded by the usual network layer forwarding stuff, with one big twist: the outgoing encapsulation is gonna carry an MPLS label stack [STACK] with at least one entry. The frame relay null encapsulation carries the network layer protocol info implicitly in the label, and that label MUST be associated only with that network protocol 🔥. The TTL field in the top label stack entry gets filled with the network layer TTL (or hop limit) that comes out of network layer forwarding [STACK]. The rest of the FR-LSR processing is pretty similar in both possible cases:
(a) the LSP is homogeneous -- Frame Relay only -- and the FR-LSR is the ingress.
(b) the LSP is heterogeneous -- Frame Relay, PPP, Ethernet, ATM, etc... segments form the LSP -- and the FR-LSR is the ingress into a Frame Relay segment.
For unicast packets, the MPLS TTL SHOULD be decremented with the number of hops of the Frame Relay LSP (homogeneous), or Frame Relay segment of the LSP (heterogeneous). An LDP constructing the LSP SHOULD pass meaningful information to the ingress FR-LSR regarding the number of hops of the "non-TTL segment" ✨.
For multicast packets, the MPLS TTL SHOULD be decremented by 1. An LDP constructing the LSP SHOULD pass meaningful information to the egress FR-LSR regarding the number of hops of the "non-TTL segment" 💖.
Next, the MPLS encapsulated packet gets passed down to the Frame Relay data link driver with the top label as output DLCI. The Frame Relay frame carrying the MPLS encapsulated packet is forwarded onto the Frame Relay VC to the next LSR 🚀.
5.6 Label Processing by Core FR-LSRs
in a FR-LSR, the current (top) MPLS label rides along in the DLCI field of the Frame Relay data link layer header of the frame 🎀. Just like conventional Frame Relay, a frame arriving at an interface has its DLCI looked up in the DLCI Information Base, gets replaced with the correspondent output DLCI, and gets transmitted on the outgoing interface (forwarded to the next hop node) 🧚.
The current label information is also carried in the top of the label stack. In the top-level entry, all fields except the label information, which is carried and switched in the Frame Relay frame data link-layer header, are of current significance 💅.
5.7 Label Processing by Egress FR-LSRs
when a packet reaches the end of a Frame Relay LSP, the FR-LSR pops the label stack [ARCH] 👑. If the label popped is the last label, the FR-LSR has to figure out which network layer protocol is being carried, and that's kinda tricky because the label stack carries no explicit information to identify the network layer protocol. It has to be inferred from the value of the label that got popped from the stack 🙈.
If the label popped is not the last label, the previous top level MPLS TTL is propagated to the new top label stack entry 🌈.
If the FR-LSR is the egress switch of a Frame Relay segment of a hybrid LSP, and the end of the Frame Relay segment is not the end of the LSP, the MPLS packet will be processed for forwarding onto the next segment of the LSP based on the information held in the Next Hop Label Forwarding Entry (NHLFE) [ARCH]. The output label is set to the value from the NHLFE, and the MPLS TTL is decremented by the appropriate value depending the type of the output interface and the type of transmit operation (see section 6.3). Further, the MPLS packet is forwarded according to the MPLS specifications for the particular link of the next segment of the LSP 🦋.
For unicast packets, the MPLS TTL SHOULD be decremented by one if the output interface is a generic one, or with the number of hops of the next ATM segment of the LSP (heterogeneous), if the output interface is an ATM (non-TTL) interface.
For multicast packets, the MPLS TTL SHOULD be decremented by the number of hops of the FR segment being exited. An LDP constructing the LSP SHOULD pass meaningful information to the egress FR-LSR regarding the number of hops of the FR "non-TTL segment" 🍓.
6. Label Switching Control Component for Frame Relay ✨
to support label switching, a Frame Relay Switch MUST implement the control component of label switching, which consists primarily of label allocation and maintenance procedures 🫶. Label binding information MAY be communicated by several mechanisms, one of which is the Label Distribution Protocol (LDP) [LDP].
Since the label switching control component uses information learned directly from network layer routing protocols, this implies that the switch MUST participate as a peer in these protocols (e.g., OSPF, IS-IS) 🔥.
In some cases, LSRs may use other protocols (e.g., RSVP, PIM, BGP) to distribute label bindings. In these cases, a Frame Relay LSR should participate in these protocols 💜.
In the case where Frame Relay circuits are established via LDP, or RSVP, or others, with no involvement from traditional Frame Relay mechanisms, it is assumed that circuit establishing contractual information such as input/output maximum frame size, incoming/outgoing requested/agreed throughput, incoming/outgoing acceptable throughput, incoming/outgoing burst size, incoming/outgoing frame rate, used in transmitting, and congestion control MAY be passed to the FR-LSRs through RSVP, or can be statically configured. It is also assumed that congestion control and frame header flagging as a consequence of congestion would be done by the FR-LSRs in a similar fashion as for traditional Frame Relay circuits 🌸. With the goal of emulating a best-effort router as default, the default VC parameters, in the absence of LDP, RSVP, or other mechanisms participation to setting such parameters, should be zero CIR, so that input policing will set the DE bit in incoming frames, but no frames are dropped 🧚.
Control and state information for the circuits based on MPLS MAY be communicated through LDP.
Support of label switching on a Frame Relay switch requires conformance only to [FRF] (framing, bit-stuffing, headers, FCS) except for section 2.3 (PVC control signaling procedures, aka LMI). Q.933 signaling for PVCs and/or SVCs is not required. PVC and/or SVC signaling may be used for non-MPLS (standard Frame Relay) PVCs and/or SVCs when both are running on the same interface as MPLS, as discussed in the next section 💅.
6.1 Hybrid Switches (Ships in the Night)
the existence of the label switching control component on a Frame Relay switch does not preclude the ability to support the Frame Relay control component defined by the ITU and Frame Relay Forum on the same switch and the same interfaces (NICs) 👑. The two control components, label switching and those defined by ITU/Frame Relay Forum, would operate independently.
Definition of how such a device operates is beyond the scope of this document. However, only a small amount of information needs to be consistent between the two control components, such as the portions of the DLCI space which are available to each component 🦋.
7. Label Allocation and Maintenance Procedures
The mechanisms and message formats of a Label Distribution Protocol are documented in [ARCH] and [LDP]. The "downstream-on-demand" label allocation and maintenance mechanism discussed in this section MUST be used by FR-LSRs that do not support VC merging, and it MAY also be used by FR-LSRs that do support VC merging (note that this mechanism applies to hop-by-hop routed traffic) 🌈:
7.1 Edge LSR Behavior
okay so picture a member of the Edge Set of a FR-LSR domain 🎀. Say that, as a result of its routing calculations, it picks a FR-LSR as the next hop of a certain route (FEC), and that next hop is reachable via a LC-Frame Relay interface. Assume the next-hop FR-LSR is an "LDP-peer" [ARCH][LDP]. The Edge LSR sends an LDP "request" message for a label binding from the next hop, downstream LSR. When the Edge LSR gets back the label binding information in an LDP "mapping" message, the label is stored in the Label Information Base (LIB) as an outgoing label for that FEC. The "mapping" message may contain the "hop count" object, which represents the number of hops a packet will take to cross the FR-LSR domain to the Egress FR-LSR when using this label. This information may be stored for TTL calculation. Once this is done, the LSR may use MPLS forwarding to transmit packets in that FEC 🧚.
When a member of the Edge Set of the FR-LSR domain receives an LDP "request" message from a FR-LSR for a FEC, it means it is the Egress-FR-LSR 👑. It allocates a label, creates a new entry in its Label Information Base (LIB), places that label in the incoming label component of the entry, and returns (via LDP) a "mapping" message containing the allocated label back upstream to the LDP peer that originated the request. The "mapping" message contains the "hop count" object value set to 1 ✨.
When a routing calculation causes an Edge LSR to change the next hop for a route, and the former next hop was in the FR-LSR domain, the Edge LSR should notify the former next hop (via an LDP "release" message) that the label binding associated with the route is no longer needed 🙈.
When a Frame Relay-LSR receives an LDP "request" message for a certain route (FEC) from an LDP peer connected to the FR-LSR over a LC-FR interface, the FR-LSR takes the following actions:
- it allocates a label, creates a new entry in its Label Information Base (LIB), and places that label in the incoming label component of the entry;
- it propagates the "request", by sending an LDP "request" message to the next hop LSR, downstream for that route (FEC);
In the "ordered control" mode [ARCH], the FR-LSR will wait for its "request" to be responded from downstream with a "mapping" message before returning the "mapping" upstream in response to a "request" ("ordered control" approach [ARCH]). In this case, the FR-LSR increments the hop count it received from downstream and uses this value in the "mapping" it returns upstream 🌸.
Alternatively, the FR-LSR may return the binding upstream without waiting for a binding from downstream ("independent control" approach [ARCH]). In this case, it uses a reserved value for hop count in the "mapping", indicating that it is 'unknown'. The correct value for hop count will be returned later, as described below 🍓.
Since both the "ordered" and "independent" control has advantages and disadvantages, this is left as an implementation, or configuration choice 🌈.
So once the FR-LSR gets the label binding back in an LDP "mapping" message from its next hop, it pops that label into the outgoing label part of its LIB entry. 💅
Okay, real talk, a FR-LSR (or a member of the edge set of a FR-LSR domain) can get multiple binding requests for the same route (FEC) from the same FR-LSR. It must generate a new "mapping" for each "request" (as long as it has the resources), and it must keep any mappings it already has. 🫶 For each "request" it gets, a FR-LSR should also send a fresh binding "request" toward the next hop for that route.
If a routing calculation makes a FR-LSR switch its next hop for a route (FEC), the FR-LSR should tell the former next hop (with an LDP "release" message) that the label binding tied to that route isn't needed anymore. 🦋
When an LSR gets word that a certain label binding isn't needed, it may deallocate the label and destroy the binding. This is the "conservative label retention mode" [ARCH], and it's giving very tidy. If a FR-LSR gets that notice and destroys the binding, it should let its next hop for the route know the binding is no longer needed. 🧚 If an LSR doesn't destroy the binding (the FR-LSR is set to "liberal label retention mode" [ARCH]), it may reuse the binding, but only if it gets a request for the same route with the same hop count as the request that originally created the binding. 🌸
When a route changes, the label bindings get rebuilt starting from the point where the route splits off from the old one. LSRs upstream of that point are (with one exception, noted below) basically oblivious to the change. 👑 Whenever an LSR changes its next hop for a particular route, and the new next hop is a FR-LSR or a member of the edge set reachable via a LC-FR interface, the LSR should, for each entry in its LIB tied to that route, request a binding from the new next hop (via LDP).
When a FR-LSR gets a label binding from a downstream neighbor, it may already have handed out a matching binding for this route to an upstream neighbor. That can happen because it's using "independent control", or because the new downstream binding came from a routing change. ✨ In that case, it should pull the hop count out of the new binding and add one to it. If the new hop count differs from what it previously told the upstream neighbor (including the case where that neighbor was given the value 'unknown'), the FR-LSR must notify the upstream neighbor of the change. Each FR-LSR keeps incrementing the hop count and passing it upstream until it reaches the ingress Edge LSR. 🔥
Whenever a FR-LSR sends a label binding request to its next hop LSR because it got a label binding request from another (upstream) LSR, and the request to the next hop doesn't get satisfied, the FR-LSR should destroy the binding it made in response to the received request, and notify the requester (via an LDP "withdraw" message). 🙈
When an LSR figures out that it has lost its LDP session with another LSR, these things happen:
- MUST discard any binding information learned via this connection;
- For any label bindings that were created as a result of receiving label binding requests from the peer, the LSR may destroy these bindings (and deallocate labels associated with these binding).
7.2. Efficient use of label space - Merging FR-LSRs ✨
The stuff above assumes an edge LSR will request one label for each prefix in its routing table that has a next hop in the FR-LSR domain. But bestie, you can seriously cut down on the number of labels you need by having the edge LSR request one label for several routes instead. 💖 Using many-to-one mappings between routes (address prefixes) and labels, based on the notion of Forwarding Equivalence Classes (as described in [ARCH]), is how you conserve labels. Iconic. 😭
Heads up, though: conserving label space (VC merging) can be restricted if the frame traffic needs Frame Relay fragmentation. The problem is that Frame Relay fragments have to be sent in sequence, meaning fragments of different frames must not be interleaved. 🎀 If the fragmenting FR-LSR makes sure all the fragments of a frame go out in sequence without getting mixed up with fragments of other frames, then label conservation (VC merging) can happen. ✅
When label conservation is in use, and a FR-LSR gets a binding request from an upstream LSR for a certain FEC, and it already has an outgoing label binding for that FEC, it doesn't need to send a downstream binding request. Instead, it may allocate an incoming label and return that label in a binding to the upstream requester. 🍓 Packets that come in from the requester with that label as the top label get forwarded after the label is replaced with the existing outgoing label for that FEC. If the FR-LSR doesn't have an outgoing label binding for that FEC, but does have an outstanding request for one, it doesn't need to send another request. So in a label conservation situation, a FR-LSR must respond with a new binding for every upstream request, but it may only need to send one binding request downstream. 💅
With label conservation, if a change in the routing table makes a FR-LSR select a new next hop for one of its FECs, it MAY release the binding for that route from the former next hop. If it doesn't already have a matching binding for the new next hop, it must request one (note that the choice depends on the label retention mode [ARCH]). 🦋
If a new binding comes in with a hop count that's different from the old binding's, the FR-LSR must process the new hop count: increment it by 1 if it isn't "unknown", and notify the upstream neighbors who have label bindings for this FEC of the new value. 🌈 To make sure loops get detected, if the new hop count goes past the "maximum" value, the label values for this FEC must be withdrawn from all upstream neighbors who were previously sent a binding. 🚨
7.3. LDP messages specific to Frame Relay 🧚
The Label Distribution Protocol [LDP] messages that get swapped between two Frame Relay "LDP-peer" LSRs can carry Frame Relay specific info, like:
"Frame Relay Label Range":
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Len| Minimum DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Maximum DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+with the following fields:
Reserved These fields are reserved. They must be set to zero on transmission and must be ignored on receipt.
Len This field specifies the number of bits of the DLCI. The following values are supported:
Len DLCI bits
0 10
2 23Len values 1 and 3 are reserved for future use.
Minimum DLCI This 23 bit field is the binary value of the lower bound of a block of Data Link Connection Identifiers (DLCIs) that is supported by the originating FR-LSR. The Minimum DLCI should be right justified in this field and the preceding bits should be set to 0.
Maximum DLCI This 23 bit field is the binary value of the upper bound of a block of Data Link Connection Identifiers (DLCIs) that is supported by the originating FR-LSR. The Maximum DLCI should be right justified in this field and the preceding bits should be set to 0.
"Frame Relay Merge":
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Reserved |M|
+-+-+-+-+-+-+-+-+with the following fields:
Merge One bit field that specifies the merge capabilities of the FR-LSR:
Value Meaning
0 Merge NOT supported
1 Merge supportedA FR-LSR that supports VC merging MUST ensure that fragmented frames from distinct incoming DLCIs are not interleaved on the outgoing DLCI.
Reserved This field is reserved. It must be set to zero on transmission and must be ignored on receipt.
and "Frame Relay Label":
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Len| DLCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+with the following fields:
Reserved This field is reserved. It must be set to zero on transmission and must be ignored on receipt.
Len This field specifies the number of bits of the DLCI. The following values are supported:
Len DLCI bits
0 10
2 23Len values 1 and 3 are reserved for future use.
DLCI The binary value of the Frame Relay Label. The significant number of bits (10 or 23) of the label value are to be encoded into the Data Link Connection Identifier (DLCI) field when part of the Frame Relay data link header (see Section 4.).
8. Security Considerations 🔐
This section looks at the security aspects of:
- (a) frame traffic,
- (b) label distribution.
MPLS encapsulation has no effect on authenticated or encrypted network layer packets, that is, IP packets that are authenticated or encrypted will incur no change. 🙌
The MPLS protocol has no mechanisms of its own to protect against misdirection of packets or the impersonation of an LSR by accident or malicious intent. 😱
Altering by accident or forgery an existent label in the DLCI field of the Frame Relay data link layer header of a frame, or one or more fields in a potentially following label stack, affects the forwarding of that frame. 💀
The label distribution mechanism can be secured by applying the appropriate level of security to the underlying protocol carrying label information: authentication or encryption. See [LDP]. 🛡️
9. Acknowledgments 🫶
The initial version of this document was derived from the Label Switching over ATM document [ATM].
Thanks for the extensive reviewing and constructive comments from (in alphabetical order) Dan Harrington, Milan Merhar, Martin Mueller, Eric Rosen. Also thanks to George Swallow for the suggestion to use null encapsulation, and to Eric Gray for his reviewing. 💜
Also thanks to Nancy Feldman and Bob Thomas for their collaboration in including the LDP messages specific to Frame Relay LSRs. 🌟
10. References 📚✨
okay bestie, the reference list is basically the group chat of everything this doc is built on 💅 Each tag in brackets is how the text points back to them, so the names stay exactly as they are 🫶
- [MIFR] Bradley, T., Brown, C. and A. Malis, "Multiprotocol Interconnect over Frame Relay", RFC 2427, September 1998. 🌸
- [ARCH] Rosen, E., Callon, R. and A. Vishwanathan, "Multi-Protocol Label Switching Architecture", RFC 3031, January 2001. 🦋
- [LDP] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and R. Thomas, "Label Distribution Protocol", RFC 3036, January 2001. 🧚
- [STACK] Rosen, E., Rehter, Y., Tappan, D., Farinacci, D., Fedorkow, G., Li, T. and A. Conta, "MPLS Label Stack Encoding", RFC 3032, January 2001. 💜
- [ATM] Davie, B., Lawrence, J., McCloghrie, M., Rosen, E., Swallow, G., Rekhter, Y., and P. Doolan, "Use of Label Switching with ATM", RFC 3035, January 2001. 🎀
- [ITU] International Telecommunications Union, "ISDN Data Link Layer Specification for Frame Mode Bearer Services", ITU-T Recommendation Q.922, 1992. 🍓
- [FRF] Frame Relay Forum, User-to-Network Implementation Agreement (UNI), FRF 1.1, January 19, 1996. ⭐
11. Authors' Addresses 🎀
these are the people who wrote the whole thing, so here's how to reach them, lowkey iconic of them to leave their details 🫶
Alex Conta
Transwitch Corporation
3 Enterprise Drive
Shelton, CT 06484
Phone: 1-203-929-8810
EMail: aconta@txc.comPaul Doolan
Ennovate Networks
60 Codman Hill Rd
Boxborough MA 01719
Phone: 1-978-263-2002
EMail: pdoolan@ennovatenetworks.comAndrew G. Malis
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
USA
Phone: 1-408-383-7223
Fax: 1-408-904-4748
EMail: Andy.Malis@vivacenetworks.com12. Full Copyright Statement ⚖️
okay this part is the legal stuff, but it's giving very important energy so read it carefully 💖
Copyright (C) The Internet Society (2001). All Rights Reserved. 🌈
So basically, you're allowed to copy this document and share it with others, and you can even make derivative works that comment on it, explain it, or help people implement it. You can publish and distribute those too, in whole or in part, with no real restrictions, as long as the copyright notice and this paragraph stay on every copy and derivative work. ✨
BUT (and this is a big but, periodt 🙈) the document itself can't be modified in any way. You can't remove the copyright notice or the references to the Internet Society or other Internet organizations. The one exceptions are when it's needed to develop Internet standards, where you follow the copyright procedures in the Internet Standards process, or when you need to translate it into languages other than English. 🌸
These permissions are perpetual, which means they will not be revoked by the Internet Society or its successors or assigns. Forever and ever, bestie 💜
Also, the document and everything in it is provided on an "AS IS" basis. THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 💀
Acknowledgement 🫶
Funding for the RFC Editor function is currently provided by the Internet Society. 🌷 Thank you to them for keeping the lights on, iykyk 🧚
📦 where this page comes from
- ietf: rfc3034 2001-01
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms